An electronic control apparatus for controlling a vehicle engine is provided with a sub-power supply circuit which always outputs the constant power supply voltage Vos based on the power of a battery, and a main power supply circuit which does not always output the constant power supply voltage Vom based on the power of the battery but only when an ignition switch of a vehicle is turned on.
The power supply voltage Vom from the main power supply circuit is supplied to a microcomputer or the like which consumes a large amount of power, while the power supply voltage Vos from the sub-power supply circuit is supplied to the circuits and memory (so-called backup RAM) or the like which must be always operated but consume remarkably small amount of power in comparison with the microcomputer or the like.
Particularly, the electronic control apparatus of this type measures, with a timer circuit which is operated by the power supply voltage Vos from the sub-power supply circuit, a period where the microcomputer is not in operation (the period in which the power supply voltage Vom is not supplied from the main power supply circuit), and activates the microcomputer with the timer circuit, when the measured period reaches the predetermined specified time, by controlling the main power supply circuit to output the power supply voltage Vom. Such timer circuit permits, even if the microcomputer is not always operated, the desired process when the specified time has passed from the timing where the ignition switch is turned off, reducing the total power consumption of the apparatus remarkably.
As the electronic control apparatus which requires such timer circuit, an apparatus for diagnosis of an evaporative gas purge system is proposed in JP-A-1996-35452.
Specifically, in the diagnosis of the evaporative gas purge system of this type, a system of engine for collecting evaporated gas (evaporated fuel generated in a fuel tank) from a fuel tank is closed for pressurization or reduction of pressure and air-tightness of this system is inspected by detecting variation of pressure in this system. Immediately after the engine has been operated for a long period of time under the heavy load condition, it is difficult to attain the accurate result of inspection because the fuel in the fuel tank easily evaporates. Therefore, when the constant period has passed from stop of engine, the air-tightness of such evaporative gas purge system is tested by the microcomputer. In this case, if the microcomputer is always operated, when the engine stops (namely, the ignition switch is turned off), to measure the constant period explained above, power consumption when the ignition switch is turned off cannot be controlled, resulting in exhaustion of battery. Therefore, when the ignition switch is turned off, supply of the power supply voltage Vom to the microcomputer from the main power supply circuit is stopped. Thereafter, the timer circuit measures the operation stop period of the microcomputer and when the measured period reaches the constant period explained above, the timer circuit causes the main power supply circuit to output the power supply voltage Vom in order to activate the microcomputer.
In the electronic control apparatus comprising such a timer circuit, if a failure that the timer circuit cannot activate the main power supply circuit (the power supply voltage Vom is outputted from the main power supply circuit) is generated because of breaking of a signal line to output the operation command signal to the main power supply circuit from the timer circuit, such failure cannot be detected. Namely, even when the period from operation stop of the microcomputer reaches the specified time of the timer circuit, the main power supply circuit cannot be operated due to a failure of timer circuit, and thereafter if the power supply voltage Vom is outputted from the main power supply circuit after the ignition switch is turned on, the microcomputer cannot detect such event and executes the processes to be executed under the normal condition.
Thus, it is impossible for the microcomputer to discriminate the condition where the ignition switch is turned on to start the operation after the specified time has passed from generation of a failure in the timer circuit and thereby the main power supply circuit cannot be operated, and the condition where the ignition switch is turned on to start the operation before the period from stop of operation reaches the specified time.